docs: initialize mdBook structure with stub pages and deployment workflow

- Add docs/book.toml with coal theme, repo link, and custom CSS
- Add docs/src/SUMMARY.md with navigation structure
- Create 16 content pages across guide/, architecture/, development/, reference/
- Add docs/theme/kindred.css with minimal sidebar overrides
- Add .gitea/workflows/docs.yml for auto-deploy on push to main
- Add docs/book/ to .gitignore

Pages are populated with real content from the codebase where possible.
Remaining pages have TODO markers for future content.

docs/theme/kindred.css

@@ -0,0 +1,8 @@
+/* Kindred Create docs - minor overrides for coal theme */
+:root {
+    --sidebar-width: 280px;
+}
+
+.sidebar .sidebar-scrollbox {
+    padding: 10px 15px;
+}